A Seminar Report OnHELICOPTER VIBRATION REDUCTION TECHNIQUES By DINU M R DEPARTMENT OF MECHANICAL ENGINEERINGVALIA KOONAMBAIKULATHAMMA COLLGE OF ENGINEERING &TECHNOLOGY PARIPPALLY,TRIVANDRUM- 691574 [2012 – 2013]
A Seminar Report On HELICOPTER VIBRATION REDUCTION TECHNIQUES In partial fulfillment of requirements for the degree of Bachelor of Technology In Mechanical Engineering SUBMITTED BY: DINU MR Under the Guidance of Shyn CS DEPARTMENT OF MECHANICAL ENGINEERINGVALIA KOONAMBAIKULATHAMMA COLLGE OF ENGINEERING & TECHNOLOGY PARIPPALLY, TRIVANDRUM- 691574 [2012 – 2013]
CERTIFICATEThis is to certify that the Seminar entitled “HELICOPTER VIBRATIONREDUCTION TECHNIQUES” has been submitted by DINU M R under myguidance in partial fulfillment of the degree of Bachelor of Technology inMechanical Engineering of Kerala University, Trivandrum during the academicyear 2012-2013 (Semester-VII).Date:Place:Guide Head, Mechanical DepartmentSHYN CS SREERAJ PS
ACKNOWLEDGEMENTApart from the efforts of me, the success of this seminar depends largely on the encouragementand guidelines of many others. I take this opportunity to express my gratitude to the people whohave been instrumental in the successful completion of this seminar.I am extremely grateful to Prof. SREERAJ PS, HOD, Department of Mechanical Engineering, forthe guidance and encouragement and for providing me with best facilities and atmosphere forthe creative work.I would like to thank my seminar guide, Mr. SHYN CS, Associate Professor, Department ofMechanical Engineering, for the valuable guidance, care and timely support throughout theseminar work. He has always a constant source of encouragement.I thank all the staff members of our department for extending their cooperation during myseminar.I would like to thank my friends for their encouragement, which helped me to keep my spirit aliveand to complete this work successfully. Dinu M R
PAGE INDEXTopic Page No.ABSTRACT1. INTRODUCTION2. OVER VIEW OF HELICOPTER VIBRATION3. HELICOPTER VIBRATION REDUCTION METHODS 3.1. PASSIVE HELICOPTER VIBRATION REDUCTION 3.2. ACTIVE HELICOPTER VIBRATION REDUCTION 3.2.1. HIGHER HARMONIC CONTROL(HHC) 3.2.2. ACTIVE CONTROL OF STRUCTURAL RESPONSE(ASCR) 3.2.3. SEMI-ACTIVE VIBRATION REDUCTION TECHNOLOGY4. COMPARISON OF THREE TECHNIQUES 4.1. PASSIVE TECHNIQUES 4.1.1. ADVANTAGES 4.1.2. DISADVANTAGES 4.2. ACTIVE TECHNIQUES 4.2.1. ADVANTAGES 4.2.2. DISADVANTAGES 4.3. SEMI-ACTIVE TECHNIQUE 4.3.1. ADVANTAGE5. CONCLUSION
FIGURE INDEXFigure Page No2.1.vibration profile of a helicopter, as a function of cruise speeds2.2. Blade Vortex Interaction (BVI) schematic3.1. Frequency response of a dynamic system with and without an absorber3.2.Boeing-Vertol CH-47 "Chinook"3.3.Sea King battery vibration absorber3.4.Parts of Vibration Reduction System3.4. Concept of HHC3.5. Individual Blade Control (IBC)3.6.Individual Blade Control (IBC) systems3.7.Basic concept of ACSR.3.8.Application of ACSR to the Westland/Augusta Helicopter5.1.Comparison of vibration levels
CHAPTER 1 INTRODUCTIONHelicopters play an essential role in today’s aviation with unique abilities tohover and take off/land vertically. These capabilities enable helicopters to carryout many distinctive tasks in both civilian and military operations.Despite theseattractive abilities, helicopter trips are usually unpleasant for passengers and crewbecause of high vibration level in the cabin. This vibration is also responsible fordegradation in structural integrity as well as reduction in component fatigue lifethe effectiveness of onboard avionics or computer systems that are critical foraircraft primary control, navigation, and weapon systems Consequently,significant efforts have been dedicated over the last several decades fordeveloping strategies to reduce helicopter vibrationA review the varioustechniques usedby different helicopter companies tocontrol helicopter vibrationsispresented here
CHAPTER2 OVERVIEW OF HELICOPTER VIBRATIONHelicopter vibration generally originates from many sources; for example,transmission, engine, and tail rotor but most of the vibration comes primarilyfromthe main rotor system, even with a perfectly tracked rotor. Fig 2.1.vibration profile of a helicopter, as a function of cruise speeds Severe vibration usually occurs in two distinctflight conditions;low speedtransition flight (generallyduring approach for landing) andhigh-speed flight.Thesevere vibration level is primarilydue toimpulsive loads induced byinteractionsbetween rotor bladesand strong tip vortices dominating therotor wake(Fig 2.2.)This condition is usually referred to as Blade Vortex Interaction (BVI).
Fig 2.2. Blade Vortex Interaction (BVI) schematic In moderate-to-high speed cruise, the BVI-inducedvibration is reducedsince vortices are washedfurther downstream from the rotor blades, and theVibration is caused mainly by the unsteadyaerodynamic environment in whichthe rotor bladesare operating.The control of vibration is importantfor four main reasons:1. To improve crew efficiency, and hence safety ofoperation;2. To improve comfort of passengers;3. To improve the reliability of avionics and mechanicalequipment’s;4. To improve the fatigue lives of airframe structuralcomponentsHence it is very important to control vibrationthroughoutthe design, developmentandin-service stages of a helicopter project
CHAPTER 3 HELICOPTER VIBRATION REDUCTION METHODS3.1 Passive Helicopter Vibration ReductionMost of the passive strategies produce moderatevibration reduction in certainflight conditions, andonly at some locations in the fuselage (such as, pilotSeats or avionics compartments) The major advantage of the passive concepts is thatthey require no externalpower to operateHowever, they generally involve a significant weightpenalty andare fixed in design, implying no ability toadjust to any possible change inoperating conditions(such as changes in rotor RPM or aircraft forwardspeed).Examples of these passive vibration reductionstrategies include Tuned-mass absorbers, Isolators Blade design optimizations. Tuned-mass absorbers Tuned-mass vibration absorbers can be employedfor reducing helicopter vibration both in thefuselage and on the rotor system. The absorbersare generally designed using classical spring masssystemstuned to absorb energy at a specificfrequency, for example at N/rev, thusreducingsystem response or vibration at the tuned frequency ( Fig 3.1.).
Fig 3.1. Frequency response of a dynamic system with and without an absorber In the fuselage, the absorbers are usually employed to reduce vibrationlevels at pilot seats or at locations wheresensitive equipment is placed.Withoutadding mass, an aircraft battery may be usedas the mass in the absorber assembly.For example, a helicopter known as seaking uses its battery vibration absorberorthe mass may be parasitic, as in certainmodels of the Boeing VertolChinookhelicopter, where five vibration absorbers one in the nose, two under the cockpit floor and two inside the aft pylon are used
Fig 3.2.Boeing-Vertol CH-47 "Chinook"Fig 3.3.Sea King battery vibration absorber
A centrifugal pendulum type of absorber mounted onthe rotor blade isanother type. This type of absorberhas been used on the Bolkow Bo 105 andHughes 500Helicopters. Next Figure shows the Hughes installation whichconsistsof absorbers tuned to the 3 And 5excitation frequencies for the four-bladedrotorversion.3.2. Active Helicopter Vibration Reduction MethodActive vibration reduction concepts have beenintroducedwith the potential toimprove vibrationreduction capability andto overcome the fixed-design drawbackof thepassive designsthe majority of the active vibration reduction concepts aimto reduce the vibration in the rotorsystem,and some active methods intend toattenuate/reducethe vibration only in the fuselage. In general, an active vibrationreductionsystem consists of four main components: Sensors Actuators Power supply unit Controller Fig 3.4.Parts of Vibration Reduction System
The principle of operation is:based on the sensor input and a mathematicalmodelof the system, generates an anti-vibration field, thatis, as closely as possibleidentical to the uncontrolledvibration field but with opposite phase. If these twovibration fields (the uncontrolled and theactuator generated) were identical inamplitude andhad exact the opposite phase, then the addition of thetwo fieldswould lead to complete elimination of thevibrations levels. Also, the controllercan be configured to adjust itselffor any possible change in operating conditionsusingan adaptive control scheme.The most commonly examined active vibrationreduction strategies include: Higher Harmonic Control (HHC) Individual Blade Control (IBC) Active Control of Structural Response (ACSR).3.2.1 Higher Harmonic Control (HHC) The main objective of this concept is to generate higher harmonicunsteadyaerodynamic loads on the rotor blades that cancel theoriginal loads responsiblefor the vibration.The unsteady aerodynamic loads are introduced by addinghigherharmonic pitch input through actuation of the swash plate athigherharmonics.The rotor generates oscillatory forces which cause the fuselagetovibrate. Transducers mounted at key locations in the fuselagemeasure thevibration, and this data is analyzed by an onboardcomputer. Based upon this data,the computer generates, using optimalcontrol techniques, signals which aretransmitted to a set ofactuators
Fig 3.4. Concept of HHC Conventionally, the swash plate is used to providerotor blade collective andfirst harmonic cyclic pitchinputs (1/rev), which are controlled by the pilottooperate the aircraft.In addition to the pilot pitch inputs, the HHCsystemprovides higher harmonic pitch inputs (for example;3/rev, 4/rev, and 5/revpitch inputs for a 4-bladedrotor) through hydraulic or electromagnetic actuators,attached to the swash plate in the non-rotating frame(Fig. 3.5.).
Fig 3.5. Individual Blade Control (IBC) The main idea of IBC is similar to that of HHC(generating unsteadyaerodynamic loads tocancel the original vibration), but with adifferentimplementation method.Instead of placing the actuators in the nonrotatingframe(HHC concept), the IBCapproach uses actuators located in the rotatingframe toprovide, for example, blade pitch,active flap, and blade twist inputs forvibrationreduction.Schematics of Individual Blade Control(IBC) systems are shown below:
Fig 3.6.Individual Blade Control (IBC) systems3.2.2 Active Control of Structural Response (ACSR)Unlike the HHC and IBC techniques that are intendedto reduce the vibration inthe rotor system, ACSRapproach is designed to attenuate the N/rev vibrationinthe fuselage, and is one of the most successfulhelicopter vibration reductionmethods at the presenttime. Vibration sensors are placed at key locations inthefuselage, where minimal vibration is desired (forexample, pilot and passengerseats or avionicscompartments). Depending on the vibration levels from thesensors, anACSR controller will calculate proper actions foractuators to reducethe vibration.The calculated outputs will be fed toappropriate actuators, locatedthroughout the airframe, to produce thedesired active forces. Fig 3.7.Shows thebasic concept ofACSR.
Fig 3.7.Basic concept of ACSR. The basis of ACSR is that, if a force F is applied to astructure at a point Pand an equal and opposite force(the reaction) is applied at a point Q, then theeffectwill be to excite all the modes of vibration of thestructure which possessrelative motion betweenpoints P and Q. This requirement for relative motion inthe model. Response between the points where the actuator forcesare applied is anessential feature of ACSR.Commonly used force actuators include: electro-hydraulic Piezoelectric, and inertial force actuators
Extensive studies on ACSR system have beenconducted analytically andexperimentally.Recently, the ACSR technology has been incorporatedin modernproduction helicopters such as the WestlandEH101 (Fig. Application of ACSR tothe Westland/Augusta Helicopter) Fig 3.8.Application of ACSR to the Westland/Augusta Helicopter
3.3. Semi-active Vibration Reduction TechnologySemi-active vibration reduction concepts aredeveloped to combine the advantagesof both purelyactive as well as purely passive concepts.Like purely activeconcepts, semi-active conceptshave the ability to adapt to changing conditions, Avoiding performance losses seen in passive systemsin “off-design”conditionsIn addition, like passive systems, semi-active systemsare considered relativelyreliable and fail-safe, andrequire only very small power (compared toactivesystems)Semi-active strategies achieve vibration reduction bymodifyingstructural properties, stiffness or damping,of semi-active actuators. Semi-activevibration reduction concepts have alreadybeen investigated in several engineeringapplicationsbut only very recently has there been any focus on using them toreduce helicopter vibration. Major differences between active and semi-activeconcepts are theiractuators and associatedcontrollers.Active actuators generally provide direct
active force,while semi-active actuators generate indirect semi activeforcethrough property modification.There are several advantages for using the semiactiveconcepts over the active concepts:power requirement of the semi-activeapproachesis typically smaller than that of the activemethods. B/c active actuatorsgenerate direct force toovercome the external loads acting on thesystem, whilesemi-active actuators only modifythe structural properties of the system. CHAPTER4
4. COMPARISON OF THE THREE TECHNIQUES4.1 Passive Techniques4.1.1 Advantages Require No external power4.1.2 Disadvantages Significant Weight Penalty Fixed in Design-no ability to adjust to any change in flight condition4.2 Active Techniques4.2.1 Advantage Low weight Penalty4.2.2 Disadvantage Requirement for external power4.3. Semi-active Technique4.3.1 Advantage
like active-adapt to changing conditions like passive- small power requirement(Compared to active) CHAPTER 5
CONCLUSIONFig 5.1.shows a comparison of the vibrationlevels of the Westland W30helicopter withouta vibration reduction system, and when fittedwith a Flexi springrotor head absorber, and anACSR system. Fig 5.1.Comparison of vibration levels REFERENCES